If human beings, human extremities or animals enter this space, body currents can be induced. Although it can be assumed that this usually does not lead to any direct hazard, particularly as there is no fear of protracted presence in this space, however the possibility of accidental exposure of children or domestic animals in particular to a strong magnetic field is likely to limit acceptance of inductive energy transmission in many population circles.
It must also be remembered that human beings can carry electrically conducting objects in the form of rings, armbands, tools, implants and the like on their body. For animals this applies in the form of collars. If such objects are exposed to a strong alternating magnetic field, eddy currents are induced in these which lead to heating dependent on the material, time of exposure, alignment in relation to the field and level of field strength. In the presence of such conditions, temperatures can be reached which can lead to damage and/or hazards. These could also emanate from electrically conductive objects which playing children might deliberately introduce into the area of the field of an inductive energy transmission device out of curiosity.
Due to the characteristics of existing applications for inductive energy transmission systems, such a hazard was assessed as not being relevant. In the case of vehicles with a driver, training can be conducted to draw attention to the need to look out for the presence of foreign bodies while in operation and to remove these before the inductive transmission is set in operation or to interrupt the inductive transmission in case of any doubt. However, the presence of foreign bodies in the area of the field appears to be an obvious safety problem for largely automatic operation or with higher safety requirements which must be assumed when such systems are used in areas accessible to the public. Here, the introduction of a foreign body while the energy transmission is operating is particularly critical, because it cannot be assumed that the driver of a vehicle will be observing the area of the field, i.e. the space between the housings of the primary coil and the secondary coil during the transmission of energy. Furthermore, the vehicle will usually be completely unsupervised during the major part of a charging operation.
From US 2007/0145830 A1 a system for the wireless transmission of electrical power to electronic devices is known, which comprises a plurality of primary coils, thereby eliminating the need for precise alignment of a primary and a secondary coil relative to each other. In this document, the problem of the presence of metallic objects is mentioned, but a metal detector is dismissed as being unusable. Rather, by suitable circuit design it is ensured that the system only enters its resonant state when a resonant circuit comprising a secondary coil and a tuning capacitor connected in parallel approaches the primary side, thereby causing a steep increase of the primary current, which is concentrated on one or a few primary coils immediately near the secondary coil. A conductive object represents no problem in this case, because it does not constitute a resonant circuit.